The present invention relates to the layering, coating and enveloping of metal parts such as tubes, pipes or other hollows, with polyethylene.
Steel pipes which are to be deposited underground require some form of insulation. A three layer synthetic jacket was found particularly suitable here. The three layers are comprised, of an adhesion enhancing cured epoxy coating being applied directly upon the steel pipe; this layer in turn is covered by an adhesion made of an ethylene copolymer and the outer jacket consists of polyethylene. Such a three layer jacket is, for example, known through the German printed Pat. No. 19 65 802, the same reference describes also a method for applying such a jacket upon the steel pipe. Basically, the steel pipe is preheated to a temperature between 140.degree. and 200.degree. C. and an epoxy resin is sprayed upon the heated pipe, the epoxy layer will cure at that temperature. As the epoxy cures, an ethylene copolymer foil or ribon is extruded and wrapped around the epoxy layer as it cures. This ethylene copolymer ribbon consitutes the adhesive proper for a polyethylene ribbon which is likewise extruded and wrapped around the adhesion carrying pipe.
This known method is not economical particularly for jacketing short tubular pieces because the composite motion various parts have to undergo during the wrapping process is quite complex. Moreover, the steel pipe may already carry on its inside a heat sensitive coating. Therefore, preheating the steel pipe for purposes of permitting the epoxy to cure may be prohibited.
German printed Pat. No. 22 22 911 describes a jacketing procedure for enveloping metal tubes with a three layer jacket under the assumption that the tube carries already a heat sensitive interior coating. The tube in this case is heated to only 70.degree. to 90.degree. C. whereupon the epoxy layer is applied. The ethylene copolymer adhesive and the outer layer material, i.e. the polyethylene, are extruded as a kind of twin hose and applied in that configuration upon the epoxy layer. The heat content of the twin hose is insufficient to provide a speedy curing of the epoxy but curing will be obtained at room temperature within about 24 hours. However, this method is likewise uneconomical or possibly even inapplicable in cases in which the hollow deviates from a cylinorical contour. Also, the rather long curing period is detrimental because of the storage requirement.
German Pat. No. 22 56 135 suggests preheating a steel pipe to 80.degree. C. prior to jacketing whereupon a epoxy resin-curing agent blend in a particular solution is electrostatically applied to that steel pipe for obtaining a coating, for example, on the order of 100 micrometers. The ethylene copolymer layer and the outer polyethylene layer are then applied together either through stretch application of an extruded twin hose or by wrapping the epoxy coated layer in extruded ribbons of the two materials. Following cooling of the jacketed pipe to a medium temperature of about 40.degree. C., the surface of the steel pipe is inductively heated to about 200.degree. C. resulting in an average temperature in the pipe of about 100.degree. C. This then permits curing of the epoxy layer within a few seconds while the interior of the pipe is comparatively little affected. Again, it has to be said that in view of the particular mode of applying the ethylene copolymer as well as the polyethylene layer, one cannot jacket noncylindrical tubular objects in this fashion.
An important feature with regard to quality of a synthetic coating or jacket is the peel strength thereof. The peel strength of a multilayer synthetic coating is to some extent detrimentally affected by interior stress and by the inherent discontinuities between the several layers. Peel tests on steel pipes which have been layered in accordance with the aforementioned methods usually exhibit a separation in the transition region, i.e. the interface zones of the several synthetic layers. The bond between the epoxy base coating and the steel pipe is usually by far the strongest. The relative peel strength as between the two thermoplastic layers, i.e. the ethylene copolymer adhesive and the outer polyethylene jacket is likewise comparatively high and can be controlled through a suitable selection of the operating and process parameters such as the temperature development. However, the peel strength is critical in the transition zone between the epoxy layer and the ethylene copolymer adhesive.